Fluid pressure control devices, such as control valves and regulators, are used in a wide number of process control system applications to control a given parameter of a process fluid (i.e., a liquid, gas, slurry, etc.). While the process control system may ultimately control the pressure, level, pH, or other desired parameter of a fluid, the device basically controls the rate of fluid flow.
Typically, a fluid pressure control device may include a valve body defining a fluid inlet passage coupled through an orifice to a fluid outlet passage. A closure member is positionable with respect to the orifice to control the amount of fluid flow therethrough. The closure member may include a valve plug having a surface which seats against a seat ring located at the orifice. A stem is attached to the closure member for driving the closure member to the desired position.
The stem typically extends through a top opening in the valve body. The top opening is closed off by a bonnet, which supports an actuator. The bonnet includes a bore through which the stem passes to allow the actuator to be operatively coupled to the stem. During operation, the process control system, or an operator manually controlling the fluid pressure control device, moves the valve plug toward and away from a sealing surface of the seat ring to provide a desired fluid flow through the orifice.
Conventional control valves typically include a packing assembly for preventing fluid leakage between the stem and the bonnet bore. An adjustable retainer assembly is often provided for compressing the packing assembly to form a sufficient seal between the shaft and bonnet. For example, commonly assigned co-pending U.S. patent application Ser. No. 10/152,602, entitled “Control Valve Stem Split Guide Bushing,” discloses a control valve having a packing retainer 46 threadably received by a lower portion 41 of a bonnet 40. A top surface of the packing retainer 46 pushes washers 49 into a packing assembly 50, thereby to compress the packing assembly. The packing retainer 46 may be threaded into the bonnet 40 to a desired position to obtain the desired compression force on the packing assembly 50.
The control valve disclosed in U.S. patent application Ser. No. 10/152,602 is suitable for many applications. Control valves having larger inlet and outlet ports, or which generate greater pressure drops, however, may generate substantial amounts of vibration that may tend to loosen the threaded connection between the packing retainer and the bonnet. As a result, the compression force on the packing assembly 50 may be reduced, thereby increasing the possibility of control fluid leaks between the stem and bonnet bore. If unchecked over a long period of time, the packing retainer may disengage the bonnet entirely, causing increased leakage and potentially interfering with the operation of the control valve.
It is also important for the stem and plug to be properly aligned with the orifice to avoid undue wear on the moving components and to ensure a sufficient seal in the fully closed plug position. Cage trims, while typically providing superior plug guidance, are not suitable for certain applications where the process fluid is gritty or sticky, and therefore a post-guided plug, such as that disclosed in U.S. patent application Ser. No. 10/152,602, may be used. The threaded connection between the packing retainer and bonnet in that post-guided plug, however, does not precisely position the packing retainer with respect to the stem, and therefore the plug and stem may be misaligned with respect to the orifice, causing increased wear of the trim components and increasing the potential for leaks.